cannot open '/etc/mail/local-host-names': World writable directory

What is World-writable?
A world-writable file or directory is one that anyone can alter. For directories this means that files can be created or deleted within it by anyone. Being world-writable is almost always a security risk.

Problem:
Get below error when try to start sendmail (/etc/init.d/sendmail start)
/etc/mail/sendmail.cf: line 94: fileclass: cannot open '/etc/mail/local-host-names': World writable directory

Solution:

1. Grant w to /etc, /etc/mail, /etc/mail/* => not work
2. Manually edit file sendmail.cf

replace line Fw/etc/mail/local-host-names with line
Fw-o /etc/mail/local-host-names

identically (if you got the same error for trusted-users)
replace line Ft/etc/mail/trusted-users with line
Ft-o /etc/mail/trusted-users